Method for producing mold for zonal optical element

ABSTRACT

A method for producing a mold for a zonal optical element having a circular portion having a circular shape taking an optical axis as a center viewed from a plane surface; at least one zonal surface formed around the circular portion in a ring form; and boundary wall surfaces connecting the circular portion with the zonal surface or connecting the zonal surfaces with each other is produced by a stage for forming a circular portion where while rotating a work around a rotation axis corresponding to the optical axis as a center, a tip is relatively moved relative to the work to form the circular portion at the mold side corresponding to the circular portion on the processing surface of the work perpendicular to the rotation axis; a stage for forming a zonal surface where the tip is relatively moved relative to the work to form a zonal surface at the mold side corresponding to the zonal surface; and a stage for forming a corner portion between the boundary portion at the mold side and the zonal surface at the mold side wherein a raked surface of the tip is in contact with a portion corresponding to the boundary wall surface.

This is a divisional of application Ser. No. 11/020,228 filed Dec. 27,2004. The entire disclosure of the prior application, application Ser.No. 11/020,228 is considered part of the disclosure of the accompanyingdivisional application and is hereby incorporated by reference.

FIELD OF THE INVENTION

1. Background of the Invention

The present invention relates to a method for producing a mold for azonal optical element, which is based on Japanese Patent Application No.2004-21439 filed on Jan. 29, 2004, disclosure of which is incorporatedherein by reference.

2. Description of Related Arts

Optical elements comprising a spherical surface having an optical axisat the center thereof and a plurality of projecting minute zonalportions formed on the circumference of the spherical surface, such asFresnel lens and blaze type diffraction element, have been produced byutilizing a mold having a concave portion corresponding to the sphericalsurface and grooves for zones corresponding to the plurality ofprojecting minute zonal portions (hereinafter referred to as “zonalgrooves) as a rule. In such a production process, it is general to cavethe minute, zonal grooves through a tip as a rule. In this process,since the edge shape of the tip is copied onto the mold, it has beendesired to point the edge shape of the tip (to sharpen the edge of thetip). However, only when the edge of the tip is sharpened, there remainsa problem in terms of changing the surface coarseness of the processedsurface for the worse. In such a situation, for example, two types oftechniques have hitherto be known as methods for producing a mold for azonal optical element.

Referring to FIG. 4, a first technique is mentioned, which is a methodin which the an edge of tip 21 serving as a cutting tool is formed intoa curve having a minute radius (minute round shape), to thereby formminute, zonal grooves onto the mold (for example, see Japanese PatentLaid-Open Publication 2003-62707). Specifically, as shown in FIG. 5,while a raked surface 21 a of the tip 21 being at a right angle relativeto a rotational direction of a column-shaped work W, the tip 21 allowsfor continuously moving along the direction of the diameter of thecolumn-shaped work W and for moving it in the shaft direction of thework W as occasion demands, whereby the edge of the tip 21 allows forfollowing up a waveform processing surface PF having been coarselyprocessed to form minutes, zonal grooves 7. This technique makes itpossible to enhance the surface coarseness and to form a bottom surfaceof each zonal groove 7 in much smaller round shape in comparison withthe case where the edge of the tip 21 is sharpened.

As a second method, a method for producing an optical element can bementioned, in which a lithographic technique utilizing many masks tofake the slope of each zonal groove in a state of steps, whereby anoptical element itself is produced (for example, see Japanese PatentLaid-Open Publication No. 9-230121). However, the production of theoptical element utilizing such a lithographic technique is not suitablefor mass production due to long processing period or such. Consequently,the optical element produced by the lithographic technique is used as amodel, and the resulting model has often been used to make a reversalprocess for producing a mold for an optical element throughelectro-forming and other forming.

However, although the first technique can slightly enhance the surfacecoarseness in comparison with the processing where the edge of the tipis sharpened, the surface coarseness is not considered to be desirablecomparing with the case of caving the processing surface by a generallyused tip. In order to enhance the surface coarseness at fullest extent,feeding speed is extremely decreased to make a process in some cases.However, this leads to increasing of processing time, as well asincreasing a possibility to changing the shape for the worse due todecreasing the temperature stability in processing environment.Furthermore, the tip is heavily worn since the edge of the tip issharpened comparing with the generally used tip.

In the reversal process in the second technique, there remains problemsin terms of technical difficulties for obtaining sufficientreversibility, and for peeling element without deformation. Moreover,when the slope is faked by steps, at least eight steps are required forthe formation of the slope. This poses problems associated with thecomplexity of aligning the mask per every steps and the like.

An object of the present invention is, therefore, to provide a methodfor producing a mold for a zonal optical element which can enhance thesurface coarseness without taking a longer period of time.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method forproducing a mold for a zonal optical element having a circular portionhaving a circular shape taking an optical axis as a center viewed from aplane surface; at least one zonal surface formed around the circularportion in a ring form; and boundary wall surfaces connecting saidcircular portion with the zonal surface or connecting the zonal surfaceswith each other. The method of the present invention comprises thefollowing stages: a stage for forming a circular portion where whilerotating a work around a rotation axis corresponding to the optical axisas a center, a tip is relatively moved relative to said work to form thecircular portion at the mold side corresponding to said circular portionon the processing surface of said work perpendicular to said rotationaxis; a stage for forming a zonal surface where said tip is relativelymoved relative to said work to form a zonal surface at the mold sidecorresponding to said zonal surface; and a stage for forming a cornerportion between said boundary portion at the mold side and said zonalsurface at the mold side wherein a raked surface of said tip is incontact with a portion corresponding to said boundary wall surface.

According to the present invention, there is also provided a mold for azonal optical element having a circular portion having a circular shapetaking an optical axis as a center viewed from a plane surface; at leastone zonal surface formed around the circular portion in a ring form; andboundary wall surfaces connecting said circular portion with the zonalsurface or connecting the zonal surfaces with each other. The mold ofthe present invention comprises a circular portion at the mold sidecorresponding to the circular portion possessed by the zonal opticalelement; at least one zonal surface at the mold side corresponding tothe zonal surface possessed by the zonal optical element; and at leastone corner portion between said boundary portion at the mold side andsaid zonal surface at the mold side wherein said corner portion is in asharpened state formed by bringing a raked surface of a tip into contactwith a portion a portion corresponding to said boundary wall surface.

According to the present invention, there is further provided a zonaloptical element produced by the mold of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a cutting lathe, which is used inan embodiment of the present invention;

FIG. 2 is an enlarged perspective view showing a tip;

FIG. 3A is a plane view showing a processing surface of work W, and FIG.3B is a cross-sectional view of FIG. 3A along with line B-B;

FIG. 4 is an enlarged perspective view showing a conventional tip whoseedge is of small round-shape; and

FIG. 5 shows the conventional method for producing a mold, where FIG. 5Ais a plane view showing a processing surface of a, and FIG. 5B is across-sectional view of FIG. 5A along with line A-A.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the attached figures as occasion may demand, embodiments ofthe present invention will be described.

As shown in FIG. 1, a cutting lathe apparatus S is mainly composed of atool portion Sa having a cutting tool 10, and a head portion onto whicha column shaped work W is fitted. As a matter of convenience, the work Wand the tip 11 are depicted at sizes different from the real sizes, andthe size relationship of them are as shown in FIG. 3.

[Tool Portion]

The tool portion Sa is mainly composed of the cutting tool 10 having thetip 10, and the cutting tool 10 is fixed onto a base (not shown) in adetachable manner.

The cutting tool 10 is mainly composed of the tip 11 and a shank 13(also referred to as a tip holder or a body of the cutting tool) ontowhich the tip 11 is fitted in a detachable manner.

As shown in FIG. 2, the tip 11 is formed into a shape where the size isnarrower at the side of the edge pushing into contact with the work W.The tip 11 may be a ready made product produced according to a generalstandard, the edge shape 11 g thereof being formed to have a relativelylarge radius of curvature.

Further explaining the constitution of the tip 11, one surface of thetip 11 constitutes a raked surface 11 a, which is mainly for cutting thework W. Also, at the edge side of the tip 11, the first escape surface11 b and the second escape surface 11 c are formed continuously with theedge shape 11 g in order to avoid interference with the work W. In thisembodiment, an escape angle (an angle relative to the base surface BF ofthe work, which will be described fully later on) of the first escapesurface 11 b is formed so as to be greater than an angle of a slope of azonal surface 61 (which will be described fully later on, see FIG. 3B)formed on the work W.

Onto the base portion of the tip 11 is perforated a bolt pore 11 h, sothat the tip 11 may be provided on a tip-holding portion 13 a of theshank 13 (see FIG. 1) by a bolt (not shown) in a detachable manner. Themethod for fixing the tip 11 onto the shank 13 is not restricted to themethod utilizing a bolt, but may be a soldering method or any of otherknown methods.

As shown in FIG. 1, the shank 11 is an arm having a substantial reverseL-shape, with the tip holding portion 13 a being formed on one endthereof. The other end of the shank 11 is fixed on a base (not shown) ina detachable manner.

[Head Portion]

A head portion Sb is mainly composed of a moving stage 31 for changingthe relative portion of the work W to the tip 11 in a three-dimensionaldirection, and a chuck 32 for rotating the work W.

The moving stage 31 possesses a moving mechanism in a longitudinaldirection, a moving mechanism in a horizontal direction, and a movingmechanism in a vertical direction (all moving mechanisms being notshown), whereby the moving stage 31 can be moved in three ways of thelongitudinal direction, the horizontal direction, and the verticaldirection. Specifically, as shown in FIG. 1 by a coordinate axis, themoving stage 31 is movable to the Z-axis parallel to the main axis lineof the head portion SB, which is the longitudinal direction; the X-axisperpendicular to the Z-axis, which is the horizontal direction; and theY-axis perpendicular to the Z-axis, which is the vertical direction.

The chuck 21 is a portion onto which the column shaped work W is fitted,and, holds the work W, for example, by means of a vacuum chuck or aplurality of jaws (also referred to as collets; not shown). The chuck 32is rotatably fitted on the moving stage 31, and rotates at a prescribedspeed by a driving motor or such (not shown). The rotation speed of thechuck 32 is a general rotation speed, which is a relatively high.

The work W is a material for molding a zonal optical element such asFresnel lens (zonal optical element having a spherical surface having acenter on the optical axis, and a plurality of projection zonal portionsformed around the spherical surface). Specifically, as shown in FIG. 3Aand FIG. 3B, a concave portion 5 corresponding to the spherical surfaceof the zonal optical element and zonal grooves 6 corresponding to theplurality of zonal portions of the zonal optical element are formed bythe tip 11. The spherical surface of the Fresnel lens corresponds to acircular potion, and each zonal portion corresponds to the zonal surfaceand the boundary wall surface. The concave portion 5 corresponds to thea circular portion at the mold side, and each zonal groove 6 correspondsto the zonal surface at the mold side and the boundary surface at themold side.

As shown in FIG. 3B, each zonal grooves 6 is mainly composed of a zonalsurface 61 (a zonal surface at the mold side) in a slope form and aboundary surface 62 (boundary wall surface at the mold side) whichstands substantially vertical direction relative to the base surface BFof the work. The term base surface BF of the work intended herein is avirtual surface perpendicular to the rotation axis T of the work W. Anangle of each zonal surface 61 (an angle relative to the base surface BFof the work) is smaller than the angle of the first escape surface 11 bof the tip 11. In convenience, from the center side to the outermostside, each of the plurality of the zonal surfaces 61 is sometimes calleda first zonal surface 61A, a second zonal surface 61B, a third zonalsurface 61C, a fourth zonal surface 61D, a fifth zonal surface 61E, anda sixth zonal surface 61F.

Subsequently, a method for producing a mold utilizing the cutting latheapparatus S will be described.

In this embodiment, prior to cutting the work by means of the tip 11,the concave portion 5 and the zonal grooves 6 have been roughlyprocessed on the work W in advance. The “rough processing” may be amethod where the shape of the processing surface 4 is roughly formed(cut) by roughly cutting the work W by means of other cutting tool, orother methods.

As shown in FIG. 1, by rotationally driving the chuck 32, the work W isrotated as a center of the rotation axis T corresponding to the opticalaxis of the Fresnel lens (see FIG. 3) and, at the same time, the work Wis moved by means of the moving stage 31, whereby the raked surface 11 aof the tip 11 is positioned at a first processing point P1 (see FIG.3A). The term “first processing point” intended herein is a positionwhich crosses a base line BL extending towards the direction of thediameter of the work W passing through center of the work W with theouter circumference of the concave portion 5 (a position slightly farfrom the outer circumference of the concave portion 5 toward a near sidein FIG. 3A). By moving the work W by means of the moving stage 31 asjust mentioned, the tip 11 is relatively moved to the work W. In thefollowing description, the description for the movement of the work Wwill be omitted and the movement of the tip 11 relative to the work willbe representatively described in convenience.

As shown in FIG. 3A, the tip 11 is moved to the direction of the arrowAR1 (left and far side in this figure, i.e., the direction along thespherical surface of the concave portion 5) to form the concave portion5 (stage for forming a circular portion). After the formation of theconcave portion 5 has been completed, the tip 11 is once released(detached) from the work W, after which the tip 11 is moved towards alower direction and a right direction, to position the raked surface 11a of the tip 11 at a second processing point P2. The term “secondprocessing point” intended herein is a position which crosses a tangentline parallel to the base line BL in contact with the outercircumference of the concave portion 5 with the outer circumference ofthe first zonal surface 61A (a position slightly far from the prescribedposition of the first zonal surface 61A toward a near side in FIG. 3A).

Then, the tip 11 is moved towards the arrow AR2 (a left direction and afar direction in the figure), i.e., the tip 11 is two-dimensionallymoved substantially parallel to the raked surface 11 a thereof, to formthe zonal surface 61 (Stage for forming a zonal surface).

Thereafter, when the tip 11 is moved to the boundary wall surface 62(the terminal of the arrow AR2), the raked surface 11 a of the tip 11 isin contact with the boundary wall surface 62 between the concave portion5 and the first zonal surface 61A to thereby form the boundary wallsurface 62. At this time, the corner portion of the zonal groove 62 isprocessed so that as if the corner portion of the zonal groove 62 iscaved by the edge portion of the raked surface 11 a of the tip 11(actually, the work W diagonally enters relative to the raked surface 11a whereby the work W is caved). Consequently, the edge shape 11 g of thetip 11 (see FIG. 2) is not copied onto the work W, the corner of thezonal groove 6 is formed in a sharpened form (the stage for forming acorner portion).

Thereafter, the tip 11 is moved to a third processing point P3, a fourthprocessing point P4, a fifth processing point P5, a sixth processingpoint P6, and a seventh processing point P7 in order to carry out thestages for forming a zonal surface (cutting along the arrows AR3 to AR7)and the stages for forming a corner portion, whereby a plurality of thezonal grooves 6 can be formed. Theses processing points P3 to P7indicate positions each crossing a tangent line parallel to the baseline BL in contact with the outer circumference of each of inner zonalsurface 61 (61A to 61E) with the outer circumference of each of outerzonal surface 61 (61B to 61F) (positions slightly far from of the secondzonal surface 61B, the third zonal surface 61C, the fourth zonal surface61D, the fifth zonal surface 61E, and the sixth zonal surface 61F)toward a near side). Specifically, in this embodiment, the tip 11 ismoved so that the raked surface 11 a is along with each tangent line ofeach boundary wall surface formed in a cylindrical shape, whereby eachzonal surface 61, each boundary wall surface 62, and each corner portioncan be formed.

As described above, the present invention has the following advantages:Bringing the raked surface 11 a of the tip 11 into contact with theboundary wall surface 62 in the stage for forming a corner portion, theedge shape of the raked surface is not copied onto the work, thus; a tip11, which is a general tip having a relatively large radius ofcurvature, can be used. This in turn makes it possible to process a moldfor producing a zonal optical element having good surface coarsenesswithout spending a long period of time. In addition, since the tip 11,which is a general tip having a relatively large radius of curvature canbe used as just mentioned, the abrasion of the tip can be reduced incomparison with a tip having a small radius of curvature as used theconventional technique.

When the concave portions 5 and the zonal grooves 6 have been roughlyprocessed prior to cutting with the tip 11, the abrasion of the tip 11,which is finally used in the finish process can be much more reduced.

Also, in the method of the present invention according to thisembodiment, since the work W can be cut only with a top portion of theedge of the tip, a tip whose top portion has been processed with highprecision can be used in the processing. Specifically, since a boarderprecision width (wind angle) can be set to be small, a mold forproducing a zonal optical element can be produced within a shorterperiod of time with high precision. In the conventional process (processshown in FIG. 5), since main parts of the edge portion should beprocessed with high precision (the wind angle must be set to be high),much more cost is required and the production period becomes longer.

It should be noted that the present invention is not restricted to theembodiment described above, and various modifications can be made. Inthis embodiment, while the boundary surface wall 62 is perpendicular tothe base surface WF of the work, the present invention is not restrictedthereto. For example, the boundary wall surface 62 may be tilted bychanging the direction of the raked surface 11 a of the tip 11 in thestage for forming a corner portion. According to this stage, thevertical wall of the mold is formed in a tapered state and, thus, azonal optical element produced can easily released from the mold. Also,the raked surface 11 a may be tilted so that the base end of the rakedsurface 11 a is approaching the processing surface 4 or the rakedsurface 11 a may be tilted taking an axis parallel to the rotation axisT as a center. The tilting angle is preferably not more than 5°, in theformer case, and is preferably not more than 3° in the latter case.

In this embodiment, while the moving stage 31, which is movable in threeaxes at a side of the head portion Sb, the present invention is notrestricted thereto. For example, it may be configured that the side ofthe head portion Sb is fixed, and a moving stage, which is movable inthree axes (longitudinal direction, horizontal direction, and verticaldirection) at a side of the tool portion Sa, or a moving stage, which ismovable in one axis (e.g., longitudinal direction) is provided at a sideof the head portion Sb, and a moving stage, which is movable in two axes(e.g., horizontal direction, and vertical direction) at a side of thetool portion Sa is provided at a side of the tool portion Sa.

In this embodiment, while the process is carried out in the order of thestage for forming a circular portion, a stage for forming the innermostzonal surface, a stage for forming the innermost corner portion, a stagefor forming another zonal surface, a stage for forming another cornerportion . . . , and a stage for forming the outermost corner portion,the order of the stages are not restricted thereto. For example, theprocess can be carried out in the reverse order, i.e., in the order ofthe stage for forming the outermost corner portion, . . . the stage forforming another corner portion, the stage for forming another zonalsurface, the stage for forming the innermost corner portion, the stagefor forming the innermost zonal surface, and the stage for forming acircular portion. In this case, the starting positions of the tip 11 forforming respective zonal grooves 6 are end terminals of AR7, AR6, AR5,AR4, AR3, and AR2 (in adjacent to the respective boundary wallsurfaces). However, the starting position of the tip 11 for forming theconcave portion 5 is preferably the starting point of the arrow AR1similar to this embodiment, because the rotation speed of the work W iszero at the end terminal of the arrow AR1.

In this embodiment, a rough processing is performed prior to thefinishing processing with the tip 11, the present invention is notrestricted thereto, and a work W having not been processed may bedirectly processed with the tip 11. In this case, boundary wall surface62 has not been formed. However, since the positions of the boundarywall surfaces and the processing positions can be set by suitablymodifying a program of NC processing apparatus (cutting lathe apparatusS), the raked surface 11 a of the tip 11 can be in contact with aposition corresponding to the boundary wall surface 62 to form thecorner portion of the zonal groove 6 in a sharpened state. Also, it isof course possible to carry out either rough processing of zonal surfaceor preferably rough processing of the corner portion.

In this embodiment, the present invention is mainly applied to aprocessing of a mold for producing a Fresnel lens having a concaveportion 5 and a plurality of zonal grooves 6, the present invention isnot restricted thereto. For example, the present invention is applicableto production of a mold for a zonal optical element having a circularportion having a circular shape taking the optical axis as a centerviewed from plane surface, and a plurality of zonal surfaces, and havinga stepped concave portion formed by proving steps (boundary wallsurfaces) between the circular portion and the adjacent zonal surface aswell as between the neighboring zonal surfaces.

In this embodiment, while the present invention is applied to a type ofthe mold where the cross-sectional shape of the zonal surface 61 is in alinear state, for example, a type of zonal optical element whose thecross-sectional shape of the zonal surface is a curved surface as shownin the drawing for prior art (FIG. 5) may also apply the presentinvention.

1. A zonal optical element produced by using a mold, having a circular portion having a circular shape taking an optical axis as a center viewed from a plane surface; at least one zonal surface formed around the circular portion in a ring form; and boundary wall surfaces connecting said circular portion with the zonal surface or connecting the zonal surfaces with each other; said mold comprising: a circular portion at the mold side corresponding to the circular portion possessed by the zonal optical element; at least one zonal surface at the mold side corresponding to the zonal surface possessed by the zonal optical element; and at least one corner portion between said boundary portion at the mold side and said zonal surface at the mold side wherein said corner portion is in a sharpened state formed by bringing a raked surface of a tip into contact with a portion a portion corresponding to said boundary wall surface. 